Modifying mode-name

[Richard Stallman]

I discovered that someone had put code in bindings.el to add text
properties to the string that the major mode put in mode-name.
These text properties were copied to other places where they did
not belong.  It was a mystery where they came from.

Please don't ever use that approach.  It is not clean, and it is not
safe, to stick text properties into strings that were set up by other
code.

I rewrote this code in a simple way that avoids modifying mode-name
itself.